1. Technical Field
The present invention relates to a device for a sound-absorbing unit. The invention is particularly intended to be utilized in connection with a sound-absorbing unit in the form of a muffler used in an exhaust system of a motor vehicle. The invention also relates to a method for affecting absorption of sound incident to gas streams such as exhaust from combustion engines.
2. Background Information
Vehicles which are driven by means of an internal combustion engine normally also contain an exhaust system which is utilized in order to guide away the discharges of the engine exhaust gases that are generated during the combustion of motor fuel. In this connection, there is a general demand for guiding the exhaust gases away from the vehicle while assuring that a minimum of noise is generated from the exhaust gas stream.
Today""s exhaust systems which are intended for use on, for example, passenger cars, comprise an exhaust pipe, at least one muffler and normally also a catalytic converter. Regarding the muffler, it is utilized to even out pulsations in the exhaust gas stream and, as a result, to make the exhaust gases as inaudible as possible. In this manner, the sound level of the exhaust gas stream can be lowered.
Apart from the requirement regarding a low sound level, it is also required in connection with today""s vehicles that the exhaust system be designed to be of small size since the available mounting room on today""s passenger vehicles is getting increasingly smaller.
In order to satisfy the requirement regarding the sound-absorbing capacity of a muffler in an exhaust system, it has been previously known to design the muffler in such a way that it can be switched between various configurations responsive to various conditions. In one case, a first condition may be characterized by low engine load. When this condition prevails, the exhaust gas stream can be guided along a certain flow path having a particularly satisfactory sound-absorbing capacity. This results in a low sound level in the exhaust system. This is particularly true when the vehicle is stationary and idling. When the first condition does not prevail, the exhaust gas stream can be guided along another flow path in the exhaust system. According to known technique, the exhaust gas stream can be guided between the different flow paths by means of an electromechanical or pneumatic regulator which is provided with an input signal from the engine. In this manner, by means of such a regulator, the gas stream can be guided between the different flow paths.
One essential drawback regarding these previously known arrangements in the relevant technical field is that there is a risk of too high of a back pressure occurring in the exhaust system in the event of a high engine load and engine speed. This may in turn result in reduced engine power, which of course is a drawback, particularly when such an operational drop occurs during high engine demand or speeds. An additional drawback is that the previously known systems require a relatively large mounting area, a condition that is particularly problematic in connection with today""s passenger cars that are growing ever smaller.
Thus, there is a need for adjustable devices for sound-absorbing units which in particular provide effective sound absorption, a small mounting volume and a low back pressure during high engine speeds.
An object of the present invention is to provide an improved device for a sound absorbing unit, which in particular is intended for an exhaust system. Another object is to accommodate a method for quieting an exhaust gas stream of a motor vehicle, by which the above-mentioned problems are solved.
A muffler device according to the present invention comprises a sound-absorbing unit for the reduction of sounds from a flowing gas stream. The muffler comprises a first flow path and a second flow path for the gas stream and a switch-over device for alternatively guiding the gas stream along the two paths. The invention is characterized in that it has a detection device for detecting the pressure of the gas stream. The switch-over device comprises an adjustable throttle adapted to be configured between closed and open orientations that respectively (1) block the first flow path when a pressure is detected that is below a predetermined limit value and (2) allow the gas stream to flow through the first flow path when the pressure limit value is exceeded.
According to a preferred embodiment, the switch-over device comprises a valve device having at least one elastic diaphragm which is acted upon by the exhaust gas"" pressure. Also, the second flow path is preferably constituted by a particular damping volume which is arranged concentrically in relation to the rest of the sound absorbing unit.
According to the invention, several advantages are achieved compared to previously known devices. First of all, it can be stated that the invention permits very effective sound absorption during low engine load, and a low fall of pressure during increasing engine load by switch-over to the above-mentioned first flow path. An additional advantage is that the invention causes a very low back pressure in the exhaust gas stream during conventional operational drops which were earlier identified with respect to high engine speeds, i.e. when the above-mentioned throttle is opened. Furthermore, due to the fact that the invention utilizes the existing pressure that prevails at the inlet to the sound-absorbing unit, excellent possibilities are provided for a simple and effective control of the position of the throttle. An additional advantage of the invention is that it requires a very small mounting space on the vehicle.
Advantageous embodiments of the invention will be apparent from the appended claims.